Probing The Excited States Of Two-Dimensional Materials Via Time-Resolved Spectroscopy

Novel two-dimensional（2D）materials have demonstrated great potential in the field of photocatalysis and nonlinear optics due to their unique optical and electronic properties.To understand the origin of their excellent opto-electronic characterisctics,the excited states of these 2D materials have to be probed.Herein we employed fast time-resolved spectroscopy to explore the excited states of black phosphorus and carbon nitrides upon photo excitation,which disentangled the photocatalytic and/or nonlinear optical response mechanism of these novel 2D materials.The study provides new insights into rational design of novel 2D photocatalyst and nonlinear materials with even better performance.The main content of the dissertation includes the following sections:1.We prepared novel iron-coordinated carbon nitride-type polymers（Fe-g-CN）under a mild one-pot method.The as-prepared photocatalyst dissolved easily in aqueous solutions and demonstrated highly efficient visible light-driven hydrogen evolution(～16.2 mmol g^-1 h^-1)with Pt nanoparticle as cocatalyst.The roles of Fe-g-CN in H2 evolution were thoroughly investigated by time-resolved spectroscopy and electrochemical measurements.It turned out that iron coordination enabled strong electron coupling between the metal and the carbon nitride and formed unique electronic structures favoring electron mobilization along the 2D nanomaterial plane,which might facilitate the electron transfer process in the photocatalytic system and lead to efficient H2 evolution.Indeed,laser photolysis measurement indicated a large electron transfer rate constant of ～5.2×104 g^-1 L s^-1 from EY to Fe-g-CN.2.We fabricated cobalt-coordinated carbon nitride-type polymers（Co-g-CN）integrating the advantages of both molecular catalytic efficiency and nano semiconductor stability,which served as homogeneous photocatalyst exhibiting superior hydrogen evolution efficiency(～12.3 mmol g^-1 h^-1)under visible light irradiation in the absence of noble metal cocatalyst.To explore the underlying photocatalytic mechanism,a series of techniques including laser photolysis,EPR,and UV-vis absorption were applied to thoroughly investigate the roles of Co-g-CN in H2 production.It turned out that cobalt intermediates（CoI,CoII,and CoIII）involved in the proton reduction by sequentially exchanging electrons,which were different from the hydrogen generation mechanism of previous carbon nitride-type catalysts.3.We employed phytic acid to facilitate the fabrication of ultrathin uniform BP QDs via liquid phase exfoliation,which exhibited superior nonlinear saturable absorption effects and a broad spectral response from the visible to the NIR region.In particular,laser photolysis measurement was performed in order to disentangle the origin of the nonlinear responses,which was related to the long-lived electron–hole pairs delocalized within the BP QD domains.